Because this way they can sell additional cards to previous PPU owners.

Yup...Good old fashion greed.

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I don't think you can derive how well gpu-x calculates physics based on a benchmark limited by fps.

And what limiting Fps in the first place?....I mean,if we need to use a faster CPU to get even higher Fps results,so that there's more physics calculations to be handled regardless of the option you pick,it kinda defeats the purpose of using it in the first place,since one of the marketing angles used for GPU physics is not having to buy the fastest CPU possible,since the the GPU is taking over the physics calculations,leaving less work for the CPU anyhow.